Application of Mineral Liberation Analysis in studying micro-sedimentological structures within sulfide mine tailings and their effect on hardpan formation

In this paper, mineral liberation analysis (MLA) was applied as a new approach to elucidate and quantify textural, mineralogical, and chemical variations on a μm/mm scale to understand the alteration processes relevant for hardpan formation at the Davidschacht mine tailings in Freiberg, Germany. Additionally, the bulk mineralogical, geochemical, and physical properties were investigated in detail. Within the upper 1.5m of this impoundment, a repetition was observed of oxidized sediments with pH values between 2 and 3, and of unoxidized sulfide/carbonate bearing sediments with pH values around 7.0. The alteration process was not homogeneous, as even within the same lamina different stages of oxidation were observed. According to the MLA measurements, the 2D pore area decreased from about 43 area-% in the unoxidized layers to 10.5-24.0 area-% in the hardpan layers, which is due to the accumulation of secondary precipitates. This clogging of pores was localized at the contact zone between two graded layers. The upper part of the lower layer consisted of very fine grained mica and chlorite fragments, which have a relatively high water retention capacity. This lamina was overlain by the bottom part of the next graded layer, which was characterized as relatively coarse grained with coarse open pores and an elevated content of primary reactive sulfides. This lamina provides access to oxygen and rain water to stimulate reactions, eventually supported by microbiological activity. Since graded layers might be amputated at top or bottom, the above described assemblage was not ubiquitous, but nevertheless multiple repetitions were encountered even at thin section level. The occurrence of these repetitions of slightly inclined tile like hardpans may reduce rain water and oxygen input at the rim of the impoundment. The application of MLA has opened up new opportunities for a better understanding of the sedimentological and mineralogical setup in relation to oxidation, transport, and precipitation processes on a μm scale within tailings impoundments.